Batteries come in a wide variety of types, chemistries and configurations, each of which has its own merits and weaknesses. Among rechargeable batteries, also referred to as secondary batteries, one of the primary disadvantages is their relative instability, often resulting in these cells requiring special handling during fabrication, storage and use. Additionally, some cell chemistries, for example lithium-ion secondary cells, tend to be more prone to thermal runaway than other primary and secondary cell chemistries.
Thermal runaway occurs when the internal reaction rate of a battery increases to the point that more heat is being generated than can be withdrawn, leading to a further increase in both reaction rate and heat generation. Eventually the amount of generated heat is great enough to lead to the combustion of the battery as well as materials in proximity to the battery. Thermal runaway may be initiated by a short circuit within the cell, improper cell use, physical abuse, manufacturing defects, or exposure of the cell to extreme external temperatures.
During the initial stages of a thermal runaway event, the cell undergoing runaway becomes increasingly hot due to the increased reaction rate and the inability of the system to withdraw the heat at a rapid enough rate. As the temperature within the cell increases, so does the pressure, leading to the eventual release of at least some of the gas generated by the reaction via the safety pressure release vent that is common to most cells. After a short period of time, the increased temperature in concert with the increased internal cell pressure leads to the formation of perforations in the cell casing. Once the cell casing is perforated, the elevated internal cell pressure will cause additional hot gas to be directed to this location, further compromising the cell at this and adjoining locations.
While the increase in cell temperature during a thermal runaway event is sufficient to damage materials in proximity to the event and to lead to the propagation of the event to adjoining cells, it is not until the hot gas escapes the confines of the cell, and potentially the confines of the battery pack, that the risk to people and property damage is significant. This is because while the event is confined, the gas generated by the event is primarily composed of carbon dioxide and hydrocarbon vapors. As a result, the autoignition temperature (AIT) of combustible materials in proximity to the event is relatively high. However, once this gas exits the confines of the cell/battery pack and comes into contact with the oxygen contained in the ambient atmosphere, the AIT of these same materials will decrease significantly, potentially leading to their spontaneous combustion. It is at this point in the event cycle that extensive collateral property damage is likely to occur and, more importantly, that the risks to the vehicle's passengers leaving the vehicle, or to first responders attempting to control the event, becomes quite significant.
Accordingly, it is desirable to detect thermal runaway events as quickly as possible, preferably before an individual thermal runaway event can propagate to other cells within the battery pack, thereby providing time to minimize property damage as well as the risk to people in relatively close proximity to the battery pack. The present invention provides a method and system for the early detection of such events.